The object of soft switch technique, as a core technique of Next Generation Network (NGN), is to build a next generation public network on a basis of packets to implement transition from Public Switch Telephone Network (PSTN) to Next Generation Network (NGN). NGN is a next-generation-oriented network and can provide an integrated opening network including a variety of services such as voice, data and multimedia. Hierarchy technique is utilized in NGN network, i.e. a whole network is partitioned into four layers, and the core techniques of the NGN network consist in service control and call control isolation.
As illustrated in FIG. 1, a soft switch system has 4 logical function layers. Different from traditional PSTN network, the four function layers, i.e. access layer, transmission layer, control layer and service layer, are completely isolated. Also, each function layer is configured using network devices having opening interfaces to form an integrated opening NGN framework.
1. Access layer: it provides a plenty of access means which connect users to a packet network and convert information format into the one that can be transmitted over a packet network;
2. Transmission layer: it provides a high-reliable, end-to-end, QoS-guaranteed, integrated transmission platform using packet technique, and provides bandwidth transmission channels for all kinds of medias.
3. Control layer: it mainly provides functions for call control and processing as well as protocols, service providing and intercommunication.
4. Service layer: it provides a plurality of network services using opening service interfaces.
In the above soft switch system, the soft switch device is control unit in a NGN network, which provides functions of call control and connection control for each device in a network, and was once called as call server, call agent or Media Gateway Controller (MGC) in different applications. Media Gateway (MG) performs overload transmission of voice streams in a network and controls conversion between one type of media stream such as Time-Division-Multiplexing (TDM)-based voice and another type of media stream such as IP/ATM data stream of IP-based Realtime Transmission Protocol (RTP). Media Gateway can be classified as Access Gateway (AGW), Trunk Gateway (TGW), Integrated Access Device (IAD) and etc. Signaling Gateway (SG) performs signaling conversion and intercommunication between a circuit switching network and a packet network and it can be a part of the access gateway. Generally, H.248 media gateway control protocol is used between a soft switch device and a media gateway, by which soft switch controls connection, establishing and release of media streams over the media gateway. The connection models defined in H.248 protocol include two main concepts, i.e. Termination and Context. Termination is a logic entity in the media gateway and can transmit and receive one or more types media streams. The main types of Termination are semi-permanent physical termination such as TDM channel or analog line, etc. and ephemeral termination such as RTP stream for carrying voice, data and video signals and all kinds of composite signals. Context indicates relations between some terminations and is a parameter describing topological relation between terminations and media hybrid/switching. It can be created by ADD command, be deleted by Subtract command and control transition of terminations between contexts by MOVE command.
FIG. 2 illustrates steps for a corresponding standard H.248 protocol controlling a trunk gateway to establish call control.
S1: The calling party dials and transmits Initial Address Message (IAM) to a soft switch device via signaling gateway SG1;
S2: The soft switch device creates a new context in TGW1 and adds TDM termination and RTP termination in the context;
S3: The soft switch device creates a new context in TGW2 and adds TDM termination and RTP termination in the context;
S4: The soft switch device transmits IAM to a circuit switching network via signaling gateway SG2, the circuit switching network reply with Address Completion Message (ACM), the called party rings;
S5: The soft switch device transmits Modify command to TGW1, notifies of a remote RTP port number and IP address, and notifies of transmitting a ring back tone;
S6: The soft switch device transmits ACM to SG1;
S7: The called party off-hooks, SG2 transmits Answer Message (ANM) to the soft switch device;
S8: The soft switch device transmits Modify command to TMG1 and cuts off the ring back tone;
S9: The soft switch device transmits ANM to SG1.
However, generally for users under control of the same media gateway, either two users under AGW (not shown) or users of two Local Exchangers (LEX) connected by the same TGW, it is needed to create two contexts to perform conversion of media streams. As shown in FIG. 3, which is a schematic diagram for end-to-end connection of carrying channels under the same trunk gateway using the standard H.248 protocol call control establishing method, when a service call such as a voice call is made between two telephone users 30, 33 on two exchangers 31, 34 connected to the same trunk gateway 36, the processing proceeds according to the above standard H.248 call control method. Calling party 30 dials and the signaling message thereof transmits Initial Address Message (IAM) to soft switch device 38 via signaling gateway 32. Soft switch device 38 needs to control trunk gateway 36 to create a new context Context Cx for calling party 30 and to add TDM termination Ttp_a and RTP termination Etp_a in the context. As such, it needs to create another new context Context Cy in the same trunk gateway 36 and to add TDM termination Ttp_b and RTP termination Etp_b in the context. TDM voice streams on calling side need to be converted into RTP streams by the media gateway, to pass through routing device 37 in a packet switching network, and to be re-converted into TDM streams, which implements normal communication between the calling party and the called party.
In the above event, for service establishment between the calling party and the called party under the same media gateway, the media gateway needs to perform conversion of media streams together with associated processing, such as digital signal processing, echo cancelling and etc., which will waste resources of the media gateway. The packet switching network needs to achieve IP loop of RTP streams, which will waste network resources.